System for ESD protection with extra headroom in relatively low supply voltage integrated circuits

ABSTRACT

An ESD protection system providing extra headroom at an integrated circuit (IC) terminal pad. The system includes an ESD protection circuit having one or more first diodes coupled in series between the supply voltage and terminal pad, and a second diode coupled to ground. One or more third diodes are coupled in series between the terminal pad and second diode, and are configured to permit a voltage on the interconnection nodes between the one or more third diodes and second diode different from ground. The one or more third diodes include an n+ on an area of P-substrate. A deep N-well separates the area of P-substrate from a common area of P-substrate, which is coupled to ground. The allowable signal swing at the terminal pad is increased to greater than supply voltage plus 1.4 V. The ESD protection circuit is useful for, among other things, relatively low supply voltage ICs.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/736,681,filed Dec. 17, 2003, now allowed, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to electrostatic discharge (ESD)protection and, more particularly, to ESD protection systems inrelatively low supply voltage integrated circuits (ICs).

2. Related Art

ESD protection circuits implemented at integrated circuit (IC)input/output (I/O) pads protect the ICs from unwanted ESD events,usually up to 2 kV. Typical ESD protection circuits are designed to turnoff during normal operations and turn on during ESD events, and arecoupled between the chip supply voltage line and ground in order toprovide an adequate discharge path.

As ICs are fabricated into the sub-micron range, supply voltages arescaled down accordingly. For example, a 0.18 μm process requires asupply voltage of approximately 1.8 V, but a 0.13 μm process requires areduced supply voltage of only 1.2 V. In low supply voltage ICs, thesignal swing at the I/O pads should be as large as possible in order tomaintain a sufficient signal-to-noise ratio (SNR).

A problem with ESD protection circuits in low supply voltage ICs islimited headroom at the I/O pads. Because the diodes in the ESDprotection circuit must remain off during normal operations in order toprevent clipping the signal, the allowable signal swing at the I/O padsis limited. For example, a typical ESD protection circuit includes afirst diode coupled between the supply voltage (V_(DD)) and the I/O pad,and a second diode coupled between the I/O pad and ground. Typicaldiodes have a forward turn-on voltage of about 0.7 V. Therefore, thediodes will remain off as long as the signal swing at the I/O pad isabove V_(DD) by approximately 0.7 V and below ground by about 0.7 V. TheESD protection circuit restrains the signal swing at the I/O pad to lessthan V_(DD)+1.4 V. If the signal swing at the I/O pad exceeds V_(DD)+1.4V, the ESD protection circuit diodes will turn on and clip the signal,reducing linearity.

What is needed is an ESD protection system that provides extra headroomat the I/O pad in relatively low supply voltage ICs.

SUMMARY OF THE INVENTION

The present invention is directed to ESD protection systems that provideextra headroom at the terminal pad for relatively low supply voltageICs. In an embodiment, an ESD protection system includes an ESDprotection circuit having one or more first diodes coupled in seriesbetween a supply voltage and a terminal pad of an IC. The ESD protectioncircuit also includes a second diode coupled to ground, and one or morethird diodes coupled in series between the terminal pad and the seconddiode. The one or more third diodes are configured to permit a voltageon interconnection nodes between the one or more third diodes and thesecond diode that is different from ground.

In an embodiment, the one or more third diodes of the ESD protectioncircuit include an n+ area on an area of IC P-substrate that issurrounded by a deep N-well. The deep N-well separates the area of ICP-substrate from a common area of IC P-substrate, which is coupled toground. In an embodiment, the forward turn-on voltages of the one ormore first diodes, the second diode, and the one or more third diodes isapproximately 0.7 V. And the ESD protection circuit increases theallowable signal swing at the terminal pad to greater than the supplyvoltage plus 1.4 V.

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to accompanying drawings. It isnoted that the invention is not limited to the specific embodimentsdescribed herein. Such embodiments are presented for illustrativepurposes only. Additional embodiments will be apparent to personsskilled in the relevant arts based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The present invention will be described with reference to theaccompanying drawings. The drawing in which an element first appears istypically indicated by the leftmost digit(s) in the correspondingreference number.

FIG. 1 illustrates an example environment in which the present inventioncan be used.

FIG. 2 illustrates a block diagram of an ESD protection system, whichlimits the headroom at the terminal pad for relatively low supplyvoltage ICs.

FIG. 3 illustrates a block diagram of an ESD protection system, inaccordance with an embodiment of the present invention, which providesextra headroom at the terminal pad for relatively low supply voltageICs.

FIG. 4 illustrates a block diagram of diode structures for an ESDprotection circuit, in accordance with an embodiment of the presentinvention, enabling multiple series diodes on a common IC substratecoupled to ground.

FIG. 5A illustrates a block diagram of an ESD protection system, inaccordance with an embodiment of the present invention, which providesextra headroom at the terminal pad for relatively low supply voltageICs.

FIG. 5B-5D illustrate example implementations of diode structures forthe ESD protection system illustrated in FIG. 5A.

DETAILED DESCRIPTION OF THE INVENTION

Overview

The present invention is directed to ESD protection systems that provideextra headroom at an IC terminal pad. In the detailed description thatfollows, an example environment in which the present invention can beused is identified and the preferred embodiments of the presentinvention are presented in detail. While specific features,configurations, and devices are discussed in detail, this description isfor illustrative purposes, and persons skilled in the art will recognizethat other configurations and devices can be used to achieve thefeatures of the present invention without departing from the scope andspirit thereof.

Example Environment

FIG. 1 illustrates an integrated circuit (IC) 100, in which the presentinvention can be used. A terminal pad 102 of IC 100 couples a mainon-chip circuit 106 to off-chip devices. If terminal pad 102 issubjected to an electrostatic discharge (ESD) event, main circuit 106can be damaged. An ESD protection circuit 104 is coupled betweenterminal pad 102 and main circuit 106 to redirect ESD away from maincircuit 106, typically to ground.

ESD Protection System with Limited Headroom

In order to describe preferred embodiments of the present invention, itis helpful to contrast the present invention with other approaches. Forexample, FIG. 2 illustrates a block diagram of an IC 200, having an ESDprotection system that limits the headroom at terminal pad 102 when asupply voltage (V_(DD)) 210 is relatively low.

IC 200 includes an ESD protection circuit 202, which is coupled betweenterminal pad 102 and main circuit 106. ESD protection circuit 202 has afirst diode 204 and a second diode 206. First diode 204 is coupledbetween supply voltage (V_(DD)) 210 and terminal pad 102. Second diode206 is coupled between terminal pad 102 and a ground 208. In anembodiment, the forward turn-on voltage of first diode 204 and seconddiode 206 is 0.7 V. First diode 204 remains off as long as a voltage onterminal pad 102 is less than supply voltage (V_(DD)) 210 plus 0.7 V.Second diode 206 remains off as long as the voltage on terminal pad 102is greater than −0.7 V. Therefore, the signal swing at terminal pad 102is limited to less than supply voltage (V_(DD)) 210 plus 1.4 V. If thesignal swing at terminal pad 102 is greater than supply voltage (V_(DD))210 plus 1.4 V, first diode 204 and second diode 208 will turn on andclip the signal, resulting in decreased linearity. For low supplyvoltage ICs, the maximum allowable signal swing at terminal pad 102 isreduced as supply voltage (V_(DD)) 210 is reduced. Therefore, ESDprotection circuit 202 provides limited headroom at terminal pad 102when supply voltage (V_(DD)) 210 is relatively low.

ESD Protection System with Extra Headroom

A potential problem with ESD protection circuit 202 is limited headroomat the terminal pad for low supply voltages. Facilitating a largersignal swing at the terminal pad of an IC is desirable in order tomaintain a sufficient SNR. FIG. 3 illustrates a block diagram of an IC300, having an ESD protection system in accordance with an embodiment ofthe present invention, which provides extra headroom at terminal pad 102when supply voltage (V_(DD)) 210 is relatively low.

IC 300 has an ESD protection circuit 302, which includes a first diode304 coupled to supply voltage (V_(DD)) 210 and coupled in series with asecond diode 306. Second diode 306 is coupled to terminal pad 102. Athird diode 308 is coupled to terminal pad 102 and coupled in serieswith a fourth diode 310. Fourth diode 310 is coupled to ground 208. Bystacking more than two diodes in series between supply voltage (V_(DD))210 and ground 208, ESD protection circuit 302 facilitates a largersignal swing at terminal pad 102 than ESD protection circuit 202, shownin FIG. 2. For example, in an embodiment of the present invention, theforward turn-on voltage of first diode 304, second diode 306, thirddiode 308, and fourth diode 310 is 0.7 V such that the signal swing atterminal pad 102 can be as large as supply voltage (V_(DD)) 210 plus 2.8V.

Structure for Series Diodes on a Common IC Substrate Coupled to Ground

A problem with stacking multiple diodes in series on a common ICsubstrate is the IC substrate is coupled to ground in order to provide acommon ground to the chip. Therefore, voltages other than ground atinterconnection nodes between the diodes coupled in series between theterminal pad and a diode coupled to ground are prevented, and theterminal pad is potentially loaded with a large parasitic cap.

FIG. 4 illustrates a block diagram of diode structures for ESDprotection circuit 302, shown in FIG. 3, in accordance with anembodiment of the present invention. The diode structures in the exampleof FIG. 4 enable stacking multiple diodes in series on a common ICsubstrate coupled to ground, while maintaining voltages different fromground at interconnection nodes between the diodes coupled in seriesbetween the terminal pad and a diode coupled to ground. In the exampleof FIG. 4, first diode 304 has a cathode 402 coupled to supply voltage(V_(DD)) 210 and an anode 404 coupled to a cathode 408 of second diode306. Second diode 306 has an anode 410 coupled to terminal pad 102.First diode 304 and second diode 306 have similar structure and areformed by p+ in an N-well 406 and 412, respectively.

Third diode 308 has a cathode 414 coupled to terminal pad 102 and ananode 416 coupled to a cathode 422 of fourth diode 310. Fourth diode 310has an anode 424 coupled to ground 208. Third diode 308 and fourth diode310 have different structures from first and second diodes 304 and 306.Third and fourth diodes 308 and 310 are formed by n+ on P-substrate.Fourth diode 310 is formed on a first area of P-substrate 426 that iscommon to an IC 400 and coupled to ground 208. Third diode 308 is formedon a second area of P-substrate 418 that is separated from first area ofP-substrate 426 by a deep N-well 420.

Deep N-well 420 isolates P-well pocket 418 and enables aninterconnection voltage between cathode 416 of third diode 308 and anode422 of fourth diode 310 that is different from ground 208. If thirddiode 308 had the structure of first and second diodes 304 and 306,terminal pad 102 would be loaded with a large N-well to P-substrateparasitic cap, potentially causing third diode 308 to turn on inresponse to a low signal at terminal pad 102.

The embodiment illustrated in FIG. 3-4 is one configuration of ESDprotection system that provides extra headroom at the IC terminal pad.Additional configurations can be implemented using more or less diodescoupled in series at an IC terminal pad so long as at least three diodesare coupled in series between the supply voltage and ground.

For example, FIG. 5A illustrates a block diagram of an IC 500 having anESD protection system, in accordance with an embodiment of the presentinvention, which has at least three diodes coupled in series betweensupply voltage (V_(DD)) 210 and ground 208. IC 500 includes an ESDprotection circuit 510 having one or more of a first diode structure 502coupled in series between supply voltage (V_(DD)) 210 and terminal pad102. ESD protection circuit 510 also includes one or more of a seconddiode structure 504 coupled in series between terminal pad 102 and athird diode structure 506, which is coupled to ground 208.

FIG. 5B-5D illustrate example implementations of diode structures forESD protection system 510, shown in FIG. 5A. In the example of FIG. 5B,first diode structure 502 is formed as p+ in an N-well 505 on a firstarea of common IC P-substrate 503. In the example of FIG. 5C seconddiode structure 504 is formed as n+ on a second area of common ICP-substrate 507, surrounded by a deep N-well 509. In the example of FIG.5D, third diode structure 506 is formed as n+ on first area of common ICP-substrate 503, which is coupled to ground 208.

ESD protection circuit 510 shown in FIG. 5 and the example diodestructures shown in FIG. 5B-5D enable coupling multiple diodes in serieson a common IC substrate coupled to ground, while allowing voltagesdifferent from ground at interconnection nodes between second diodestructures 504 and third diode structure 506. Therefore, ESD protectioncircuit 510 provides extra headroom at terminal pad 102 when supplyvoltage (V_(DD)) 210 is relatively low.

CONCLUSION

The present invention has been described above with the aid offunctional building blocks illustrating the performance of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed. Any such alternate boundaries are thus within the scope andspirit of the claimed invention. One skilled in the art will recognizethat these functional building blocks can be implemented by discretecomponents, application specific ICs, processors executing appropriatesoftware and the like or any combination thereof.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. Thus, the breadth and scope of thepresent invention should not be limited by any of the above-describedexemplary embodiments, but should be defined only in accordance with thefollowing claims and their equivalents.

1. An ESD protection circuit, comprising: a first set of diodeselectrically coupled in series between a supply voltage and a terminalpad, each diode in said first set of diodes being placed in its ownrespective well on a common area of a substrate; and a second set ofdiodes electrically coupled in series between the terminal pad andground, wherein a first diode in said second set of diodes is coupled toground and sits directly, without an intervening well, on said commonarea of said substrate and wherein subsequent diodes in said second setof diodes are placed in respective areas of said substrate electricallyisolated from said common area of said substrate.
 2. The ESD protectioncircuit of claim 1, wherein said respective areas of said substrate areisolated by deep wells from said common area of said substrate.
 3. TheESD protection circuit of claim 1, wherein an allowable signal swing atsaid terminal pad is greater than said supply voltage plus 1.4 V.
 4. TheESD protection circuit of claim 1, wherein a forward turn-on voltage ofeach diode of said first and second sets of diodes is approximately 0.7V.
 5. An ESD protection circuit, comprising: first and second diodeselectrically coupled in series, wherein a cathode of said first diode iscoupled to a supply voltage and an anode of said second diode is coupledto a terminal pad, said first and second diodes placed in respectiveseparate wells on a substrate; a third diode having a cathode coupled tosaid terminal pad, said third diode placed on an electrically isolatedarea of said substrate, said electrically isolated area separated by adeep well from other areas of said substrate; and a fourth diode placeddirectly, without an intervening well, on said substrate, said fourthdiode having a cathode coupled to an anode of said third diode and ananode coupled to a ground.
 6. The ESD protection circuit of claim 5,wherein said first diode and said second diode each includes an n+ and ap+ diffusion region in said respective wells on said substrate.
 7. TheESD protection circuit of claim 5, wherein an interconnection nodebetween said third diode and said fourth diode is isolated from saidground.
 8. The ESD protection circuit of claim 5, wherein said substrateis coupled to said ground.
 9. The ESD protection circuit of claim 5,wherein an allowable signal swing at said terminal pad is greater thansaid supply voltage plus 1.4 V.
 10. The ESD protection circuit of claim5, wherein a forward turn-on voltage of each of said first diode, saidsecond diode, said third diode, and said fourth diode is approximately0.7 V.
 11. The ESD protection circuit of claim 5, wherein said thirddiode includes an n+ and a p+ diffusion region in said deep well. 12.The ESD protection circuit of claim 5, wherein said fourth diodeincludes an n+ and a p+ diffusion region on said substrate.
 13. The ESDprotection circuit of claim 5, further comprising a plurality of fifthdiodes coupled in series between said third diode and said fourth diode,said fifth diodes placed on respective electrically isolated areas ofsaid substrate, said respective electrically isolated areas separated byrespective deep wells from said other areas of said substrate.
 14. TheESD protection circuit of claim 1, wherein an interconnection nodebetween one of said subsequent diodes and said first diode is isolatedfrom said ground.